Au/Y觸媒應用於室溫一氧化碳氧化反應

Abstract

本研究以氯化金酸為前驅物的奈米金擔載於HY型(proton type)沸石來製備Au/Y觸媒，用於催化室溫下的一氧化碳氧化反應。研究分為三部分，第一部分，探討表面修飾對於沸石表面電位的影響。由於沸石的isoelectric point (IEP)為小於pH 3左右，在中性環境下表面電位為負，因此不適用於擔載同樣帶負電荷的金化合物於沸石擔體上，然而，透過在硝酸鈉溶液中的表面修飾程序，沸石的表面電位可由負變正。因此，在第二部分的研究中，就以氯化金酸為前驅物的奈米金擔載其上，探討擔體的表面電位改變，對於奈米金的載負以及催化活性的影響。由研究結果發現，隨著表面電位由負變正，金的擔載量隨之提升，此外，由於金顆粒小同時擔載位置接近外表面，因此具有較佳的催化活性。另一方面，透過降低金溶液的pH值至2.6，亦可使表面電位由負變正，然而，低pH下金化合物的氯殘留量較高，因而對觸媒的催化活性造成負面影響。總結而言，在接近中性環境下(pH 6)，將奈米金擔載於經過表面修飾的HY上，具有最佳的催化一氧化碳氧化反應活性。在本研究第三部分，則探討水氣對Au/Y觸媒催化活性的影響。由實驗結果發現，在完全隔絕水氣的環境下，Au/Y幾乎沒有活性，顯示水氣對催化活性的必要性。透過改變水氣含量，探討反應溫度攝氏25度時濕度對催化活性的影響，已結論出相對濕度為30 %時，Au/Y觸媒具有較佳的活性。

Nanogold derived from chloroauric acid (HAuCl4) were deposited onto proton-type zeolite Y (HY) for catalyzing CO oxidation around room temperature. Three subjects were studied in this research. In the first part, the effect of surface modification on surface charge of proton-type zeolite Y (HY) was investigated. Owing to the negative zeta potential in neutral aqueous solution close to neutral, zeolite Y is not suitable for supporting negative-charge gold complexes. Nevertheless, the process of surface modification in NaNO3 solutions, with solution pH adjusted to 6 by NaOH(aq), was found able to reverse the zeta potential of HY to positive. In the second part of this study, the effect of surface charge of zeolite supports on the Au loading and catalytic activity was investigated. Positive correlation was observed between the surface charge and Au loading. For modified HY with positive surface charge, the supported gold species were with smaller sizes and finer dispersion. Through ESCA analyses, most of them were deposited near the exterior surface, which enables less resistance in mass transfer for CO oxidation. They possessed catalytic activity superior to those supported on HY without any surface modification. On the other hand, via lowering the solution pH to 2.6 (near IEP of zeolite supports), the surface charge can be reversed to positive as well. However, more residual Cl remained inside the catalysts. Moreover, gold species were deeply deposited inside the cages of HY. The as-prepared Au/Y possessed almost no catalytic activity. In summary, it is recommended to prepare Au/Y in aqueous solution at pH 6, with surface modified HY with positive surface charges. In the third part of this study, the effect of moisture on the catalytic activity of Au/Y was investigated. The results demonstrate that moisture is indispensable for Au/Y to maintain high catalytic activity. Under a relative humidity at ca. 30 % at 25 °C, Au/Y catalysts can perform the best activity in this research.